Polyimides are known in the art for use in the manufacture of integrated circuits including chips (e.g. chip back end of line), thin film packages and printed circuit boards. Polyimides are useful in forming dielectric interlayers, passivation layers, alpha particle barriers and stress buffers. Radiation sensitive polyimides are particularly useful as an interlayer dielectric material to insulate the conductor wiring interconnecting the chips on a multichip module. This is known as thin-film wiring. A multichip module is an intermediate level of packaging between the chips and the circuit board. Multichip modules are generally known in the art. Multichip modules are made up of multiple layers of power, signal and ground planes which deliver power to the chips and distribute the input/output signals between chips on the module or to/from the circuit board.
Radiation sensitive polyimides can also be used as the dielectric material for insulating the conductor wiring on a silicon carrier as a multichip module. The use of radiation sensitive polyimide provides a simplified method for the fabrication of the patterned polyimide required for these thin film wiring schemes. It is important that the final properties of the patterned polyimide are not significantly altered relative to the nonradiation sensitive polyimides also used in forming the multichip module.
Radiation sensitive polyimides useful as a dielectric interlayer for conductor wiring in a multichip module are known in the art. There are two general types of radiation sensitive compositions used to form a patterened polymide film. The first type of composition comprises an unimidized polyamic acid ester having a crosslinkable moiety or an unimidized polyamic acid salt having a crosslinkable moiety, a radiation sensitive initiator and a sensitizer. The sensitizer functions to transfer energy from incident radiation to the radiation sensitive initiator. These compositions are generally first patterned and then imidized. For example, "Polyimide Coatings" by Craig, Electronic Materials Handbook, Vol. 1, discloses negative tone radiation sensitive polyamic acid esters where the ester moiety is covalently bound to the polymer backbone and contains unsaturated linkages in the form of acrylates or methacrylates. Upon exposure to light, these groups, in conjunction with a suitable radiation sensitive initiator and a sensitizer, crosslink via a free radical mechanism resulting in differential solubility between the exposed and unexposed regions. Subsequent development with a suitable solvent system followed by imidization results in a negative tone imaged polyimide dielectric interlayer. An example of the polyamic acid salt radiation sensitive composition is disclosed in Yoda et al., "New Photosensitive High Temperature Polymers for Electronic Applications," J. Macromol. Sci.-Chem., A21(13&a4), 1641 (1984). Yoda discloses incorporating in the polymer an alkyl ammonium salt as a radiation sensitive group. The alkyl ammonium salt is formed from the reaction of a suitable poly(amic acid) with a tertiary amine having unsaturated groups in the form of acrylates or methacrylates.
The second type of radiation sensitive composition comprises soluble, fully imidized polyimide having crosslinkable groups, a radiation sensitive initiator and a sensitizer. Upon exposure to radiation, the crosslinking reaction introduces differential solubility in the polyimide film which can then be developed with organic solvents. Examples of this type of radiation sensitive composition are disclosed in J. Pfeiffer et al., "Direct Photoimaging of Fully Imidized Solvent-Soluble Polyimides, "Proc. Second Intern. Conference on Polyimides, Ellenville, N.Y. (1985) which is incorporated by reference. Such radiation sensitive compositions are also commerically available such as from Amoco Corporation under the trademark Ultradel.
To form a film of the radiation sensitive composition in the manufacture of integrated circuits, it is necessary to dissolve the composition in a suitable organic solvent having acceptable properties. Unfortunately, polyimides, polyamic acid salts and polyamic esters are generally insoluble in most organic solvents. However, many polyimides and polyamic acid salts and esters are soluble in N-methylpyrrolidone (NMP) which is particularly useful in the manufacture of integrated circuits. Therefore, it is desired that the radiation sensitive initiator and the sensitizer each have good solubility in NMP. The good solubility enables the composition to be formed as a homogeneous film in the manufacture of integrated circuits and also enables the storage of the composition at a low temperature prior to use without precipitation of the initiator or the sensitizer.
Thioxanthone sensitizers are known in the art, as for example, disclosed in Fischer et al., U.S. Pat. No. 4,681,950. However, the thioxanthones disclosed by Fischer form a gel in the lithographic composition, or have relatively low solubility in NMP. The low solubility of the thioxanthone in NMP causes precipitation of the thioxanthone during storage prior to its use in manufacturing. Therefore these thioxanthones are not suitable for use in radiation sensitive compositions used to form a patterned polyimide film in the manufacture of integrated circuits.
It is therefore an object of the present invention to provide an improved sensitizer for use in radiation sensitive polyimide compositions.